Conventionally, removable adhesive sheets have been used so far for surface protection and prevention of damages during manufacture of parts to be fabricated finely such as semiconductors, circuits, various kinds of printed substrates, various kinds of masks, and lead frames. The removable adhesive sheets used for these purpose are required to be non-contamination type so that adherends are not contaminated with organic materials or particles attributable to adhesive compositions. While solvent type acrylic adhesives have been used so far for such application, since the solvent type acrylic adhesives are synthesized in organic solvents, evaporation of the solvents during coating causes an environmental problem and conversion of them to water-dispersible acrylic adhesives has been intended. However, since the water-dispersible acrylic adhesives use emulsifiers, it has been difficult to decrease contamination in comparison with the solvent type acrylic adhesives.
Particularly, in the adhesive sheet for use in semiconductor wafer processing used in the processing application of semiconductor integrated circuits, contamination-free property is important. Usually, a semiconductor integrated circuit has been manufactured by slicing, for example, silicon single crystals at high purity into wafers, incorporating an integrated circuit by etching to form a predetermined circuit pattern such as an IC to the surface of the wafer, then polishing the wafer rear face by a grinder to decrease the wafer thickness as thin as 100 to 600 μm and, finally, dicing the same into a chip. During the polishing, an adhesive sheet is bonded to the wafer surface for preventing damages of the wafer or facilitating the polishing work. Further, during the dicing, an adhesive sheet is bonded on the side of the wafer rear face and the wafer is subjected to dicing while being adhered and secured, and the formed chip is picked-up by knocking up on the side of a substrate by a needle and fixing the same on a die pad.
The adhesive sheet for use in semiconductor wafer processing used for such a purpose is required to have such an adhesive force as not to be removed during the polishing or dicing and, on the other hand, to have such a low adhesive force that it can be removed easily upon pick-up after the polishing or dicing and also it does not damage the semiconductor wafer. Further, the adhesive sheet is required not to cause adhesive residues on the wafer surface or the wafer rear face so as not to contaminate the surfaces. Particularly, along with the increase in the density and the performance of semiconductor integrated circuits in recent years, administration for the contamination to semiconductor wafers and circuit surfaces of semiconductor chips obtained therefrom has become severer. Accordingly, further less-contaminating property has been demanded to adhesive films for use in the fabrication of wafers more than even before.
Further, it has been known that contaminants on the wafer surface give undesired effects on the shear strength of a wire bonding. That is, in the wire bonding conducted upon manufacture of semiconductor chips, while it is necessary that the adhesive force between a ball and a pad is high, organic materials or particles adhered on the aluminum surface over the wafer result in a factor of inhibiting the adhesion of the gold wire to the aluminum surface, and in a case where a great amount of contaminants are deposited on the aluminum surface, they result in a problem of lowering the wire bonding shear strength.
On the other hand, unevenness such as a pattern or a dicing line is present on a wafer to be polished. The adhesive sheet is bonded to the surface of the wafer pattern having such unevenness. In a case where the adhesive sheet does not fill the unevenness on the wafer surface and voids are present, or in a case where the adhesion to the wafer surface is poor, water intrudes during polishing to result in a problem of contaminating the surface of the wafer pattern. While a removable water-dispersible acrylic adhesive sheet, which includes an adhesive layer formed of a water-dispersible adhesive with less contamination to the surface of an adherend, and is usable, particularly, as an adhesive sheet for use in semiconductor wafer processing, has already been proposed (JP-A-2003-82307), compatibility between high resistance to water intrusion and less-contaminating property has been demanded further.